In designing printed microwave circuits of the stripline or microstrip variety it is often necessary to couple a D.C. bias potential to a transmission line. Such D.C. coupling is required, for example, for controlling switching diodes included in the rf circuitry. Switching diodes are employed in such rf circuits as step attenuators, switches, limiters, amplifiers and phase shifters.
Heretofore, a satisfactory D.C. bias port has been provided through the use of the conventional high impedance grounded quarter-wavelength stub. The quarter-wavelength stub when connected to the rf transmission line presents essentially infinite impedance to the rf signal and thus provides an ideal circuit path for a D.C. bias signal, the D.C. bias being applied to the stub conductor at the ground point thereon. However, when the rf transmission line to which the quarter-wavelength stub is connected has to handle a signal bandwidth exceeding approximately one half octave, substantial rf signal loss is encountered at the D.C. bias port. One way of extending the bandwidth capacity of the quarter-wavelength stub bias circuit has been to fabricate the stub in the form of the so-called "flying lead." A flying lead is a quarter-wavelength stub which is supported above the insulative substrate of the rf circuit such that its characteristic impedance is increased. However, since the characteristic impedance of the lead is still finite, the line length remains a significant factor such that a significant amount of "tweaking" of the elevated conductor element is necessary to obtain satisfactory operation of the circuit over, for example, a two octave bandwidth. As a result of such hand tuning, flying lead circuits are expensive to manufacture and almost impossible to produce with a high degree of repeatability.
Other techniques for securing broadband, high impedance grounded bias ports have been through use of printed lumped inductors and cylindrical inductors which are mounted as discrete components on the rf circuit substrate. These techniques also require high tolerance, expensive manufacturing procedures and thus, from that standpoint, have the same drawbacks as flying lead circuits.
Accordingly, it is an object of the present invention to provide a low cost, easily manufacturable printed rf circuit for coupling a D.C. bias signal to an rf transmission line with minimum attenuation of the rf signal over a bandwidth exceeding two octaves.
Another object is to provide a low cost, broadband printed rf circuit of the type described which is fabricated solely through use of standard microstrip or stripline circuit printing processes.
In accordance with the invention a highly effective printed D.C. bias port is provided for an rf transmission line by connecting a grounded high impedance stub conductor to an area of the rf transmission line which is configured into a plurality of series-connected conduction segments having different line length and impedance values. The line length and impedance parameters of the series-connected transmission line segments and the stub conductor are selected to provide a maximum voltage transmission coefficient across the portion of the transmission line including the series-connected conductor segments.
In accordance with another aspect of the invention there is provided a method for rapidly calculating the optimum line length and impedance parameters for the conductor elements.
These and other objects, features and advantages will be made apparent by the following detailed description of a preferred embodiment of the invention, the description being supplemented by drawings as follows.